Pupal-puparial cuticle from Dipterans is stabilized by synthesis of two conjugates, protein-aryl-protein and chitin-protein. The identity of the components in each complex is unknown. The linked proteins will be identified by larval-pupal sequence homologies in species where the arthropodins are closely related or identical with the respective sclerotins in primary structures. Benzenoid-linked regions will be identified by prior administration of the bridge precursors, 14C-Ring-L-tyrosine and 3H-lysine, affording labelled cross-linked peptides on fragmentation with formic-methane sulfonic acid, cold N-bromosuccinimide and alkaline-borohydride. Two sequence probes which are not affected by the resistance of these entities to enzymatic degradiation will be exploited for N- and C-termini, namely the methods of Edman and Stark-Kassell. Homologous peptides will be generated from untanned larval proteins purified by chromatography and iso-electric focussing using the glutamyl-aspartyl specific protease from S. aureus. Since the lysyl component of the bridge is included within the primary structure, homologous regions in the unsclerotized larval proteins will be labelled by administration of radio-lysine early in the last instar to restrict the sequencing to the appropriate peptides. The structure of the glycosylated bridge will encompass sequencing of the peptide component pre-labelled with glutamic acid and glycine, residues characteristic of the two developmental stages in question.